Transmission and Distribution: Complete Guide to Power System Basics | By Stupid Engineers

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Transmission and Distribution: Complete Guide to Power System Basics

Electricity is one of the most essential resources in modern life, and its efficient transmission and distribution are critical for reliable power supply. In this article, we’ll break down the fundamentals of the electrical power system, voltage levels, line classifications, statutory regulations, and distribution system components — all explained in a clear, SEO-friendly way.

1. Fundamentals of the Electrical Power System

An electrical power system consists of three main components:

  1. Generating Stations – Facilities where electrical power is produced. Examples include thermal, hydro, and nuclear power plants.
  2. Transmission Lines – High-voltage networks that transport bulk electricity over long distances from generating stations to load centers.
  3. Distribution System – A network that delivers electricity from the transmission lines to consumers at usable voltage levels.

2. Standard Voltage Levels and System Structure

Generation Voltage

Electric power is typically generated at 11 kV.

Transmission Voltages

  • Primary Transmission – Voltage is stepped up for long-distance transport to minimize losses. Common voltage levels: 66 kV, 132 kV, 220 kV, 400 kV, and 765 kV.
  • Secondary Transmission – Voltage is stepped down to 33 kV for further distribution, usually as a 3-phase, 3-wire system.

Distribution Voltages

  • Primary Distribution – Stepped down to 11 kV at substations, then carried by feeders in a 3-phase, 3-wire system.
  • Secondary Distribution – Stepped down to the final consumer voltage:
    • 400V / 415V (3-phase, 4-wire) – Industrial and commercial loads.
    • 230V / 240V (1-phase, 2-wire) – Residential and light commercial loads.

3. Classification of Lines and Cables

Overhead (OH) Lines by Voltage

  • Low Voltage (LV) – < 250 V
  • Medium Voltage (MV) – 250 V – 650 V
  • High Voltage (HV) – 650 V – 33 kV
  • Extra High Voltage (EHV) – 66 kV, 132 kV, 220 kV
  • Ultra High Voltage (UHV) – 765 kV and above

Underground Cables by Voltage

  • Low Tension (LT) – Up to 1 kV
  • High Tension (HT) – 1 kV – 11 kV
  • Super Tension (ST) – 11 kV – 33 kV
  • Extra High Tension (EHT) – 33 kV – 66 kV
  • Extra Super Tension (EST) – 66 kV – 132 kV

Specialized Cable Types

  • Belted Cables – Used up to 11 kV, sometimes up to 22 kV.
  • Screened Cables (H-type & S-L type) – Used from 22 kV to 66 kV.
  • Pressure Cables (Oil or Gas filled) – Used for voltages above 66 kV.

4. Statutory Regulations (IE Rules)

IE Rule 54 – Voltage Variation Limits

Defines permissible voltage fluctuation at the consumer’s terminal:

  • Low & Medium Voltage (up to 650 V): ±6%
  • High Voltage (up to 33 kV): +6% to −9%
  • Extra High Voltage (above 33 kV): +10% to −12.5%

IE Rule 55 – Frequency Variation Limit

The declared supply frequency should not vary by more than ±3%.

5. Distribution System Components

  • Feeder – Connects substation to distribution area. Carries constant current with no intermediate tappings. Designed for current-carrying capacity.
  • Distributor – Conductor from which multiple tappings supply consumers. Designed for permissible voltage drop limits.
  • Service Mains – Cable connecting the distributor to the consumer’s meter, designed based on load capacity.

6. Economical Transmission Voltage

An empirical formula gives the economical line voltage (V in kV):


V = 5.5 \times \sqrt{\left(\frac{L_{\text{km}}}{1.6} + \frac{P_{\text{kW}}}{100}\right)}
  • L = Distance in km
  • P = Power in kW

Rule of thumb: 650 V/km (0.65 kV/km) is a good preliminary estimate.

7. Kelvin’s Law for Conductor Size

Kelvin’s Law determines the most economical cross-sectional area of a conductor for minimizing the total annual cost of energy loss and capital investment.

Conclusion

The transmission and distribution system plays a vital role in ensuring a stable, efficient, and safe power supply. From generation at power stations to delivery at homes and industries, every component — from feeders and distributors to cables and statutory regulations — is designed for reliability and efficiency.

By understanding these fundamentals, electrical engineers, students, and technicians can design and maintain better power systems for the future.

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